1. Field of the Invention
This invention relates to mobile radio systems. The invention will be described primarily in relation to cellular radio networks. However the invention is also applicable to other mobile radio systems, such as private mobile radio (PMR) systems.
2. Related Art
In a typical cellular radio network a number of radio base stations are located throughout the area in which radio coverage is required, in order to allow mobile units throughout that area to be in radio communication with the fixed part of the network via one of the radio base stations. The radio base stations comprise radio transceivers for establishing radio communication with nearby mobile units. Several radio channels are provided to allow simultaneous communication with several mobile units. These radio channels may for example be separate time slots in a time-division scheme, and/or different radio frequencies. The radio base stations are themselves physically connected by fixed links to a switching centre, and thereby a communications link can be activated between two mobile units via respective radio base stations, or between a mobile unit (again via a radio base station) and a fixed telephone network e.g. PSTN, or other cellular radio system, or other telecommunications network.
The radio base stations require control functions to establish radio communication with mobile units, and to carry out various other functions to determine which mobile units are within its area of coverage in order to direct incoming calls to the correct mobile unit, and to arrange handover of calls should a mobile unit move during the course of a call from the area served by one base station to that served by another. Such control functions include commands to the radio base stations to communicate with a mobile unit on one of its allocated channels, including instructions as to when to start and terminate the call, or to carry out handover processes.
In the GSM (Global System for Mobile radio) standard, these control functions of the radio base stations are functionally, and usually physically, separate from the transceivers they control. The control functions are performed by a "Base Site Controller" (BSC) controlling the radio transceivers of several radio base stations, (known in the GSM system as "Base Transceiver Sites" (BTS)), which may be some distance away. Because of the complexity involved, it is advantageous to concentrate the necessary equipment to perform the control functions in a small number of locations in this way, to provide ease of access for maintenance. The terminology used in the GSM standard is used herein for convenience, but is not limitative on the scope of the claims. In particular, it should be noted that the term "radio base station" embraces a base transceiver site unless the context clearly demands otherwise.
The connections between the base transceiver sites and the base site controller can be quite long, typically several tens of kilometers. The connections between the base transceiver sites and the base site controllers can make up a substantial part of the infrastructure of the cellular radio network. In many cases the fixed links from several base transceiver sites meet at some point, hereinafter referred to as a "branch point", intermediate between the base transceiver site and the base site controller or other switch, and continue from the branch point over a common trunk link, for the rest of the route to the base site controller or other switch.
Each base transceiver site has a number of radio channels available to it. The number of channels determines the maximum number of mobile units which can communicate with a base transceiver site simultaneously. In order that this maximum capacity can be achieved, the fixed physical link between the base transceiver site and the base site controller requires at least the same number of individual communication channels to be available to it. The term "channel", as used herein, refers to the resources (time slot, cable, carrier frequency etc) used to carry an individual call over the communications link or links in question. A "radio channel" is such a channel in a radio link, and similarly for a "trunk channel" etc.
For that part of the routing between the various base transceiver sites and the base site controller which is shared over a common trunk link, the number of channels required in the fixed link is equal to the total capacity of all the base transceiver sites. This is wasteful of the capacity of the link over the common trunk, because it is most unlikely that all the base transceiver sites will be operating at full capacity at the same time.
Even in systems in which the control functions and radio transceivers are physically co-located in a radio base station, (that is to say, the base site controller functions are carried out at the base transceiver site) a similar problem arises in the fixed links between the base stations and the main mobile switching centre (MSC), which serves many radio base stations. The fixed links also form a branched network and there can be over provision of capacity in the trunk common to several branches for the same reasons.
International patent specification WO94/00959 (Nokia) describes an arrangement in which a synchronous digital hierarchy (SDH) network comprising a loop serves a number of individual microcell base transceiver sites. At each base transceiver site on the loop there is an Add-drop multiplexer (ADM)which allows the channels relevant to the base transceiver site or sites served by the multiplexer to be extracted. Since the SDH loop is common to all the base transceiver sites, and any of its channels may be allocated to any of the base transceiver sites according to demand, fewer channels are required in the SDH system than the total combined capacity of the base stations.
However, this system suffers from a number of disadvantages. Firstly, the basic element of an SDH carrier, known as STM-1, has a capacity of 155 Mbit/s. A typical microcell site requires only 320 kbit/s. Consequently, to use the SDH network to capacity, more than 500 microcell sites would have to be served by each loop if the total combined capacity of the base stations is to be greater than the capacity of the SDH system. This is an inefficient use of the capacity of the SDH system, as each microcell on the loop has to be fed by two 155 Mbit/s connections, in order to supply a 320 kbit/s capacity. Moreover, five hundred microcell sites would serve a large area, and to have a large area served by a single loop would leave it very vulnerable to any faults--two faulty links could isolate all five hundred microcells. Also, the physical size of an add-drop multiplexer is very much greater than that of the microcell base site electronics itself, so such a microcell/ADM combination would be less convenient to install, and have greater visual impact. Moreover, the arrangement described in the above-mentioned patent specification has a single base site controller (BSC) and mobile switching centre serving all the base transceiver sites. This requires control signals to be transmitted over the SDH loop between the BSC and each BTS. A channel for such control signals, to control handover etc, must be available to each base transceiver site, even when not in use, so that a handover can be initiated. Each channel of each base site transceiver has its own signalling channel, and these would all have to be forwarded over the SDH loop to the BSC.